1. Field
One or more embodiments relates to a photon counting system and method.
2. Description of the Related Art
In general, a photon counting detector is a detector capable of counting a single photon of incident light. For example, the photon counting detector may include a photomultiplier, which typically has a photocathode and an electron multiplier, composed of several dynodes, and an anode. As photons of light enter the photocathode, photoelectrons are emitted by the photocathode toward the electron multiplier, for a secondary electron emission, and then collected by the anode. Accordingly, charges are generated at the anode in the form of current pulses, with each pulse generally representing a single photon count, i.e., if a photon was collected by the photocathode and then multiplied by the electron multiplier then a corresponding pulse of an expected height and width should be observed at the anode. By reviewing the charge at the anode, as each expected pulse is detected the total photon count can be incremented. Accordingly, a photon counting detector is configured to detect photons of incident light and output pulse signals corresponding to the detected photons. Typically, the number of pulse signals corresponds to the number of photons.
As shown in FIG. 6A, to count the photons, the output pulse signals need to be subject to signal processing, such as a current to voltage converter to convert the current pulse signals at the anode into voltage signals, an amplifier to amplify the voltage signals, and a discriminator to discriminate between pulse heights that are too low, thereby representing a non-photon event from some noise, or pulse heights that are too high, potentially representing other inaccuracies. Finally, after the discriminator there may be a pulse shaper to reshape the output signal of the discriminator, as the output of the discriminator may be a constant level, such as a TTL logic level of CMOS level. Thus, the pulse shaper may form a rectangular pulse that is more preferable to the counter for counting the discriminated pulses. The counter may then count the number of pulses, represented by the rectangular pulse that are passed through the discriminator.
However, for such signal processing there is an accompanying dead time, i.e., a period of time when the system is currently processing an operation, such as another counting of a photon from a previous pulse signal, and can therefore not immediately commence with processing another predetermined instruction, such as a processing of a most recent pulse signal that is output from the photomultiplier during this dead time, without inaccuracies being generated in the counting of photons.
Thus, when considering the dead time in a system, if a second pulse signal is output by the photomultiplier, for example, indicating the detecting of a second photon by the photomultiplier while a first pulse signal corresponding to a first photon is being still being processed, a “pile-up” where the first and second pulse signals overlap each other occurs. FIG. 6B illustrates such a pile-up situation, where the charges pile up through the addition of several pulses output by the photomultiplier
Such a pile-up prevents an accurate counting of photons. Conventionally this problem has been alleviated by attenuating or eliminating the second pulse signal by use of a rejector, i.e., the second pulse to be applied to the signal processing is rejected if received during the dead time. However, when this solution is employed, photons that are detected by a photomultiplier, for example, during the dead time are not counted by the adjoining signal processing elements, they are rejected or eliminated. This can be seen in FIG. 6C, where until the charge from the first pulse has adequate time to dissipate additional charges from subsequent pulses are not added, resulting in the photon counts from those subsequent pulses being lost. Therefore, this solution of rejecting potentially properly detected photons, while alleviating the pile up problem, is not suitable for application in photon counting, as the total number of counted photons will be inaccurate.